Vitamin D and Exercise: Deficiency Signs, Testing and Food Sources

Vitamin D is one of the most important micronutrients for active people, and deficiency is remarkably common in Iran. Despite plenty of sunshine in most cities, apartment living, long hours indoors, covering clothing and air pollution mean many people, athletes included, have low levels. In this article we explain what vitamin D does for exercise, the signs of deficiency, which blood test to ask for, and which foods can help.

What does vitamin D do in the body?

Vitamin D behaves more like a hormone than a typical vitamin. Your skin makes it when exposed to UVB light from the sun, and the liver and kidneys then convert it into its active form. Its key roles include:

• Bone health: it helps absorb calcium and phosphorus from the gut; without it, even plenty of dairy won't fully protect your bones.
• Muscle function: muscle tissue has vitamin D receptors, and severe deficiency is linked to muscle weakness, especially in the thighs and hips.
• Immune function: adequate levels support a normally working immune system, which matters for people with a high training load.
• Injury prevention: long-term deficiency is associated with a higher risk of stress fractures, particularly in runners and military recruits.

Why are Iranian athletes at risk?

You might assume that someone who trains and looks healthy has enough vitamin D, but that is often not the case. Common factors in Iran include:

1. Indoor gyms: most training happens indoors or in the evening, when there is effectively no UVB.
2. Clothing: when only the face and hands are exposed, the skin makes little vitamin D.
3. Sunscreen and glass: sunlight through a window does not produce vitamin D.
4. Air pollution: in cities like Tehran, Isfahan and Ahvaz, particulate matter absorbs some UVB.
5. Darker skin, excess weight and age: all three reduce production or availability of vitamin D.
6. Little oily fish: oily fish is not a regular part of many family diets.

Signs of vitamin D deficiency

The main problem is that deficiency is often silent, and its symptoms are vague and non-specific. Possible signs include:

• Persistent fatigue and low energy even with enough sleep
• Dull bone aches, especially in the lower back, hips and shins
• Muscle weakness or unexplained loss of strength
• Slow recovery and prolonged muscle soreness after training
• Frequent colds and respiratory infections
• Stress fractures or bone injuries from minor loads
• Low mood, particularly in autumn and winter

All of these can have other causes, such as anaemia, an underactive thyroid, poor sleep or overtraining. That is why the only reliable way to know is a blood test, not guessing from symptoms.

The vitamin D test: what to measure

The standard test of your body's stores is 25-hydroxyvitamin D in the blood. Most labs in Iran offer it, and fasting is usually not required. Many references interpret results roughly like this:

Reference ranges differ slightly between labs, and the final interpretation should come from a physician who considers your age, conditions and medications. They may also check calcium, phosphorus or parathyroid hormone. After starting treatment, a repeat test after about three months is commonly advised to adjust the dose.

Vitamin D supplements: only with a physician

Very high-dose vitamin D products are easy to buy in Iran, and many people take them without testing, on a friend's advice. That is risky. Vitamin D is fat-soluble and stored in the body; too much can raise blood calcium and cause nausea, intense thirst, kidney damage and kidney stones. Therefore:

• Let a physician decide the dose and duration based on your test result.
• A treatment dose differs from a maintenance dose; don't continue a treatment dose on your own.
• If you have kidney disease, kidney stones, a parathyroid condition or are pregnant, speak with your physician before doing anything.
• Take vitamin D with a meal that contains some fat for better absorption.

Food sources of vitamin D

Few foods are naturally rich in vitamin D, and reaching an optimal blood level from diet alone is usually difficult, but food plays an important supporting role. Options available in Iran:

• Oily fish: trout, salmon, sardines and canned tuna. Two servings a week is a good target.
• Egg yolks: the amount is small but useful alongside other sources; free-range hens produce richer yolks.
• Fortified dairy: some milks and yoghurts on the market are fortified with vitamin D, so read the label.
• Mushrooms: mushrooms exposed to sunlight or UV light produce some vitamin D. Try leaving sliced mushrooms in midday sun for 15–30 minutes.
• Liver: contains a little vitamin D, but because of its high vitamin A content it shouldn't be eaten in large amounts.

A sample day: two eggs with sangak bread for breakfast, grilled trout with vegetables for lunch, and a glass of fortified milk as a snack. Besides vitamin D, this combination provides plenty of protein for muscle repair. To set your daily calories for your goal, use the calorie calculator.

Sunlight: the main free source

For most people, the sun is the most important source of vitamin D. As general guidance, exposing your arms and legs to direct sunlight for about 10–30 minutes around midday, a few times a week, helps many people with light to medium skin. Those with darker skin need longer. At the same time, avoid sunburn, since it raises the risk of skin cancer. A practical approach for athletes is to move part of your training outdoors: brisk walks, easy runs in the park or bodyweight work in the yard. The ProGym exercise library has dozens of equipment-free movements that suit outdoor training.

Vitamin D, calcium and training: how they work together

Vitamin D alone doesn't build bone; it also needs enough calcium and the mechanical loading that exercise provides. Weight-bearing activities such as brisk walking, easy running, skipping rope, squats and deadlifts signal your bones to become stronger. Most adults are advised to get around 1,000 mg of calcium a day, which three servings of dairy (a glass of milk, a bowl of yoghurt and a matchbox-sized piece of cheese) plus leafy greens, sesame and almonds largely cover.

If a test shows severe deficiency and you have bone pain or muscle weakness, reduce high-impact training during the first weeks of treatment and ask your physician when it is safe to return to heavy work. An online coach can adjust your programme during this period so you keep your fitness without overloading your bones.

Summary

Vitamin D deficiency is common in Iran, even among athletes, and can come with fatigue, muscle weakness, bone pain and stress injuries. Rather than guessing or self-prescribing high-strength supplements, get a 25-hydroxyvitamin D test and let a physician set any dose. Alongside that, add regular sunlight, oily fish, eggs and fortified dairy to your routine so your bones and muscles are better prepared for training.

Frequently asked questions